1. Field of the Invention
The present invention relates to a control system for a fuel cell to generate an electric power by an electrochemical reaction between a fuel gas and an oxidizing gas and, more particularly, to a control system for controlling a humidification of the fuel cell.
2. Related Art
The fuel cell of a polymer electrolyte film type or a phosphoric acid type has to be supplied from the outside with water for keeping the electric conductivity or activity of the electrolyte. In order to lower the electric resistances of electrodes, on the other hand, the fuel cell has to be held in a wet state. For example, the former fuel cell using the polymer electrolyte film such as an ion exchange film is humidified by supplying it with a fuel gas (e.g., a hydrogen gas) and an oxidizing gas (e.g., air) containing water, so as to increase the electric conductivity of the electrolyte film and decrease the electric resistances of the individual electrodes.
The humidification is desired to be as much as possible for improving the electric conductivity or the electric resistances. If the water supply is so excessive that the electrolyte or the electrodes are wetted with water droplets, however, the gas reaction is obstructed to lower the output of the fuel cell. On the other hand, the hydrogen ions (or protons) having passed through the electrolyte film may combine with the electrons on the cathode side to cause the hydrogen gas to be mixed into the air. Therefore, the humidification is restricted from the viewpoint of preventing the flooding and the accompanying reduction in the output. Thus, it is the general practice to control the humidification of the fuel cell not only on the basis of a predetermined target value but also on the basis of the electric resistance or output detected.
In the invention disclosed in Japanese Patent Laid-Open No. 10-255828, on the contrary, the humidification is performed by controlling the injection rate of pure water into the supply gas on the basis of the flow rate and temperature of the supply gas, the temperature of the pure water, the load on the cell and the internal temperature of the cell.
In the aforementioned fuel cell of the solid polymer film type, for example, the water to be supplied to the electrolyte and the individual electrodes is the one supplied together with the gases, the one to penetrate due to the humidity difference and the one generated by the reaction. On the contrary, the water to be discharged is the one to be discharged together with the gases. The water on the supply side is controlled, in the invention disclosed in the Laid-Open, on the basis of the supply gas flow rates, the pure water temperature, the cell load and the cell internal temperature. This is the control of the water volume, as demanded for running the fuel cell. In addition to this control, on the other hand, the disclosed invention prevents the water from becoming relatively excessive to cause the dew condensation, by controlling the water supply on the basis of the supply gas temperature.
In the running state of the fuel cell, the supply and the discharge of water concurrently occur. In the disclosed invention, however, the water supply is controlled, but the water to be discharged together with the excess gas is not controlled. In other words, the internal situation of the fuel cell is not necessarily precisely reflected in the humidification control, and the water content may become excessive or short.
A main object of the invention is to control the water content in a fuel cell while reflecting the internal situations of the cell more precisely.
Another object of the invention is to provide a control system capable of generating a high power stably.
We have noted that the absolute water content in the fuel cell becomes the more at the higher internal pressure to make it hard to cause the water shortage by the excess gas carrying away the water, while the absolute water content becomes the less at the lower internal pressure to make it easy to cause the water shortage by the excess gas carrying away the water. Therefore, the invention is characterized by adopting the pressure in the fuel cell as the control parameter for the humidification.
According to the invention, there is provided a control system for a fuel cell to output an electric energy by a reaction between a fuel gas and an oxidizing gas, comprising: means for decreasing the humidification of the fuel cell when the pressure in said fuel cell is high, and for increasing the humidification of the fuel cell when the pressure in said fuel cell is low.
When the absolute water content in the fuel cell is high because of the high internal pressure and when its shortage is hardly caused by the gas carrying away the water, according to the control system of the invention, a new water supply to or humidification of the fuel cell is decreased to prevent the excessive water content. When the absolute water content in the fuel cell is low because of the low internal pressure and when its shortage is easily caused by the gas carrying away the water, on the contrary, a new water supply to or humidification of the fuel cell is increased to prevent the water shortage.
In the control system of the invention, on the other hand, the humidification for each drive situation is determined on the basis of a physical model of said fuel cell to be controlled, for which the water balance is predetermined.
In the control system thus constructed, it is possible to set the humidification reflecting the running situation of the fuel cell more precisely.
Moreover, the control system of the invention can further comprise: means for detecting the internal resistance of said fuel cell; and means for correcting the humidification determined by reflecting the running situation on the basis of the detected internal resistance.
In the control system thus constructed, the humidification can be performed without raising the internal resistance thereby to raise the output of the fuel cell.
In addition to the aforementioned individual constructions, moreover, the control system can further comprise: means for detecting the pressures in the entrances of said gases to be fed to said fuel cell; and means for correcting the humidifications determined by reflecting the running situation on the basis of the detected entrance pressures.
According to the control system thus constructed, when the entrance pressure of the gas of the fuel cell is high, the gas having the water content hardly enters the fuel cell. Since the humidification is corrected on the basis of the entrance pressure, however, it is possible to avoid the short humidification, even when the gas hardly enters the fuel cell, and the excessive humidification even when the gas easily enters.